[FuncSpec] Fix missed opportunities for function specialisation

When collecting the possible constant arguments to
specialise a function the compiler will abandon the search
on the first argument that is for some reason unsuitable as
a specialisation constant. Thus, depending on the traversal
order of the functions and call sites, the compiler can end
up with a different set of possible constants, hence with
different set of specialisations.

With this patch, the compiler will skip unsuitable
constants, but nevertheless will continue searching for
more.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D135867